Worldwide ~140 million people are chronically exposed to arsenic (As) at concentrations that exceed the WHO guideline for safe drinking water. Arsenic causes many adverse health outcomes, including several types of cancer. However, the underlying mechanisms are unknown. Although eliminating As exposure remains the primary goal for preventing As toxicity, cancer risks persist decades after As exposure has been reduced. Therefore, understanding the mechanisms by which As causes human diseases, such as cancer, is critical for developing future intervention strategies to reduce As toxicity. Though As is not mutagenic, an increasing body of literature suggests that As causes epigenetic dysregulation, including altered histone methylation patterns. Thus, dysregulation of histone methylation may be one mechanism by which As causes cancer. Currently, little is known about histone methylation in human populations exposed to As-contaminated drinking water. Importantly, S-adenosylmethionine (SAM) provides the methyl groups for histone methylation, and S-adenosylhomocysteine (SAH) is a potent product inhibitor of SAM-dependent methylation reactions; since SAM and SAH are both tightly regulated by folate-dependent one-carbon metabolism, folate nutritional status may also influence histone methylation through its effects on SAM and SAH. However, there is a lack of information in the scientific literature on relationships between SAM, SAH, folate, and histone methylation in As-exposed populations. For this study, [three] histone modifications (H3K36me2, [H3K36me3], H3K79me2) were selected, because they are known to be influenced by As and/or methyl donors in vitro. The first aim of this project is to test the following hypotheses: 1) As exposure increases global [and gene-specific] levels of histone mark H3K36me2, [and causes a corresponding decrease in H3K36me3], in peripheral blood mononuclear cells (PBMCs) from Bangladeshi adults who are chronically exposed to As- contaminated drinking water, 2) the global increase in H3K36me2 is mediated by a decrease in histone lysine demethylase 2B (KDM2B) mRNA expression, and 3) H3K36me2 [and H3K36me3] levels persist even after As exposure has been reduced through the use of water filters that remove As. The second aim of this project is to test the hypotheses that folate nutritional status, SAM, SAH, and folic acid (FA supplementation, influence global [and gene-specific] levels of H3K36me2, [H3K36me3], and H3K79me2 in PBMCs from As-exposed Bangladeshi adults enrolled in the Folic Acid and Creatine Trial (FACT), which is a double-blind, randomized, placebo-controlled trial of FA and/or creatine supplementation. Better understanding the relationships between As, folate, and histone methylation will increase our knowledge of the mechanisms by which As causes human diseases, and will improve our ability to design future intervention strategies, such as nutritiona interventions, to prevent or reverse epigenetic dysregulation and thereby reduce disease burden in populations exposed to As-contaminated drinking water.